Active laser fibers are used to generate laser light. Such devices are also called fiber lasers. Laser fibers typically have a doped core that is the active medium that guides the laser light. An outer pump excitation fills the complete fiber diameter and interacts with the active fiber core. Due to the multiple reflections within the core along the length of the fiber, a large gain in the laser radiation, which is coupled out at the end of the fiber, is possible.
To achieve a high yield in conversion of pump energy to laser power, active fibers containing rare-earth elements are used. The active fibers are surrounded with a pump core which is characterized by a disturbed cylindrical symmetry or by mechanical tension applied by differences in refractive index within the pump core. Conventional methods for production of these pump cores include using a cylindrical preform and creating a symmetry disturbance by the grinding of segments. The resulting pump cores exhibit a D-structure or double-D-structure or a multiple W-structure. The grinding of the preform is a separate manufacturing process that consumes resources and time.
Conventional methods for more economical production of active laser fibers include the following:
U.S. Pat. No. 6,411,762 discloses a laser fiber optimising the mode mixing by implementing doped rods around the active core. For this purpose, a multimode preform is prepared by producing holes with an ultrasonic treatment to insert the doped rods in a subsequent process step.
U.S. Pat. No. 4,978,377 discloses a production method for polarization-maintaining fibers that combines different cladding segments by conservation of their symmetry. The ultrasonic grinding as well as the combining of cladding segments is typically quite costly and generally needs a high level of technological experience.
It remains desirable to have an efficient method for making active laser fibers.